C-4 Boiler, Hot Water Heating and Steam Fitting Contractor license in California

Updated November 1, 2026 · Official name from CSLB's classification list

• 708licenses hold C-4
• 96.9%Clear (CA addresses)
• 16.3%workers' comp exempt (all: 49.6%)
• 100%contractor's bond on file
• 26.1 yrsmedian time licensed
• 81.5%businesses, not sole owners

C-4 is the license for boilers and the pipes, valves and radiators that move heated water or steam through a building. It covers installing, servicing and repairing power boilers and low-pressure hot-water heating boilers, plus the pumps, gauges, convectors, fuel-oil tanks and lines, chimneys and flues tied to them. Solar heating equipment on these systems is included.

You are most likely to need a C-4 in an older home or apartment building with radiators, or for radiant hydronic heat. Forced-air furnaces are a different trade.

Typical work under C-4

• Replacing a hot-water or steam boiler
• Radiators, baseboard convectors and hydronic piping
• Steam fitting, valves, gauges and circulation pumps
• Fuel-oil tanks and supply lines for a boiler
• Boiler flues and chimney connections
• Solar heating equipment tied to a hot-water heating system

Work that needs a different license

• Forced-air furnaces and central air conditioning: C-20 HVAC
• A standard tank or tankless water heater for faucets and showers: C-36 Plumbing
• Running a new gas line to the boiler room: C-36 Plumbing

Tips before hiring

• Ask whether the crew has worked on your type of system. Steam and hot-water boilers behave differently.
• Get the boiler make, model and efficiency rating written into the contract.
• Ask how the old boiler will be removed and whether the insulation on old pipes will be checked for asbestos first.

Where C-4 contractors are

The cities with the most C-4 license holders in CSLB's current file:

1. Boiler and Steam Fitting Contractors in San Francisco: 45
2. San Jose: 16
3. Los Angeles: 15
4. Santa Rosa: 15
5. San Diego: 14
6. Fresno: 12
7. Sacramento: 12
8. El Cajon: 10
9. Bakersfield: 9
10. Hayward: 8

All boiler and steam fitting contractors lists

Cities with at least 20 C-4 license holders, largest first:

• San Francisco (45)

How to verify a C-4 license

1. Search the business or license number here and confirm C-4 is listed on the profile.
2. Check the same number on CSLB's Instant License Check.
3. Read our full guide to checking a contractor license.

Related license types

• C-20 Warm-Air Heating, Ventilating and Air-Conditioning Contractor
• C-36 Plumbing Contractor
• C-38 Refrigeration Contractor
• C-22 Asbestos Abatement Contractor

Source: CSLB public data · Updated November 1, 2026 Status can change at any time. Verify on CSLB's Instant License Check.

More license types: ← C-2 Insulation and Acoustical Contractor · C-5 Framing and Rough Carpentry Contractor →